This invention relates to a heat-sealable wrapping material and in particular to such material having a non-blocking surface overcoating applied thereto. More specifically, the invention relates to a wrapping material including a flexible packaging material substrate such as paper, cellophane, or a composite laminated web coated on one side with a thermoplastic heat-sealable wax and elastomer composition which is normally somewhat tacky at room temperature and finally a non-blocking overcoating composition superposed on the surface of the heat-sealable wax composition.
Flexible packaging sheet materials employing a surface coating of a composition including a substantial percentage of petroleum waxes of both paraffin and microcrystalline types have long been utilized in the packaging industry because of their excellent combination of protective properties, low cost, freeness from taste and odor, heat-sealability and excellent appearance in terms of gloss and luster. Wax compositions utilized in many packaging applications include various modifying additives such as natural and synthetic rubbers, resins, polymers and the like, which alter many of the physical and protective properties of the coating in adapting it to specific applications. Of particular interest in the present invention are wax compositions adapted to the manufacture of coated wrappers for cheese and cheese products, the wrappers bearing coatings comprising petroleum waxes, primarily of the mirocrystalline type, and a small percentage of an elatomer such as polyisobutylene, generally ranging from 3 to 20% of the total coating weight. These coatings, when applied to a flexible base sheet such as cellophane, metal foil or combinations of these materials with other sheet materials, yield heat-sealable wrappers having excellent protective properties for cheese packaging. The surfaces of the coatings are, however, slightly tacky at room temperature and the wrappers tend to stick together when sheets are stacked one on another or when a coated sheet material is stored in roll form. This sticking or "blocking" of the coated sheet materials presents a severe problem in the use of these wrappers and has necessitated the application to the coating surface of heretofore a release-treated interleave sheet, a dusting of a solid particulate, pulverulent tack-relieving material or an overcoating of a non-blocking composition applied in the form of a solution or suspension in a volatile organic solvent. Although all of these procedures have been more or less successful in relieving the blocking problems, they have suffered from one or more undesirable features including inconvenience, a deleterious effect on the heat-sealing qualities or transparency of the wrapper, or have involved the necessity for the removal of large quantities of volatile organic solvents, with the attendant problems relating to fire and health hazards and environmental pollution.
It is therefore an object of the present invention to provide an improved packaging material for use on cheese and other food items, the said packaging material having a non-blocking overcoating applied from an aqueous dispersion onto the surface of a normally tacky wax composition coated flexible packaging base sheet.
It is a further object to provide a flexible, wax composition coated cheesewrapper having a non-blocking overcoating of excellent transparency and heat-sealability without the use of substantial amounts of solvents which present hazards both to health and to the environment.